Dose effect on mechanical properties of high-energy nitrogen implanted 316L stainless steel

In this work, we investigated the influence of nitrogen dose on mechanical properties of 316L stainless steel. Implantation energy was fixed at 1 MeV and the fluence was varied between 5×1016 at cm−2 and 1018 at cm−2. The current density and the sample temperature during treatment were kept at constant values, 1.5 μA cm−2 and Timp=−30 °C, respectively. Nano-indentation tests show improved mechanical properties. Maximum hardness values linearly increased with increasing implantation dose, from 5.25 to 7.5 GPa. However, hardness profiles indicate that these values are highly affected by the unimplanted substrate just beneath the implanted layer. Nano-scratch tests confirm the previous results, showing an increase of dynamic elastic recovery (Rd) and a significant decrease of the dynamic friction coefficient (μd) from 0.55 to 0.35. These improvements of mechanical properties are correlated with the surface modifications: results of micro-structural analysis through grazing incidence X-ray diffraction (GIXRD) show iron-nitride formation (ε-Fe2N and ε-Fe3N) and high level of residual stresses. The variation of the glancing angle indicates that the residual stress is non-homogeneously distributed through the depth.